Stiffness of Base Plate on Perimeter Wall

[SteelPE]

I am trying to figure out the stiffness of a base plate for a steel column resting on a perimeter foundations wall. I am running through the procedure in the PCI Design Hand Book (section 3.8.2). Using the PCI I can easily calculate the flexibility of the base plate and the anchor bolts but I’m not sure how to account for the flexibility of the foundation wall/footing.

The PCI does give an equation for the flexibility of the foundation but that is for an isolated footing only.

 

[civilperson]

Flexibility is the inverse of stiffness, EI is the stiffness. The foundation wall will have much greater stiffness, (order of magnitude greater).

 

[SteelPE]

I understand that flexibility is the inverse of stiffness. In the PCI they have an equation for the flexibility of the footing = 1/(soil modulus * moment of inertia of footing)esentially 1/EI. This is for an isolated footing. For a column that is going to be on a foundation wall would you assume then that the foundation is infinitely rigid because the moment of inertia for the wall footing would be incredibly large?

 

[msquared48]

If you are talking about a long stemwall, 8" to 10" thick, four to 8 feet high...most definitely assume infinitely rigid.

However, if you are talking about a short 6" thick stemwall, one foot or less high, over a thin strip footing, I would look to the spread footing comparison.

Anything in between, assume something between a spread footing and infinity. That close enough for you?

Mike McCann
MMC Engineering